The present invention relates to electromagnetic radiation detection systems. In particular, the present invention is directed to arrays of photoconductive detector elements.
When radiation of the proper energy falls upon a semiconductor, the conductivity of the semiconductor increases. Energy supplied to the semiconductor causes covalent bonds to be broken, and electron-hole pairs in excess of those generated thermally are created. These increased current carriers decrease the resistance of the material. This "photoconductive effect" in semiconductor materials is used in photoconductive detectors.
A photoconductive detector is a bar of semiconductor material having electrical contacts at the ends. In its simplest form, the photoconductive detector is connected in series with a direct-current voltage source and a load resistor. The change in resistivity of the photoconductive detector in response to incident radiation is sensed in one of two ways. If the resistance of the load resistor is much greater than the resistance of the detector, the device is operating in the "constant current mode," since the current through the detector is essentially constant. In this mode, the change in resistivity of the photoconductive detector is usually sensed by measuring the voltage across the photoconductive detector.
If, on the other hand, the resistance of the load resistor is much less than the resistance of the detector, the photoconductive detector is operating in the "constant voltage mode," since the voltage across the photoconductive detector is essentially constant. The change in resistivity of the photoconductive detector is usually sensed by measuring the voltage across the load resistor.
Of the two detector modes, the constant current mode finds wider use in photoconductive detectors made from semiconductor materials having low resistivity. For this reason, further discussion in this specification will deal with the constant current mode rather than the constant voltage mode.
Photoconductive detectors, and particularly arrays of photoconductive detectors, have found many applications. One particularly important area is in the detection of infrared radiation. Infrared sensitive photoconductive detector arrays are used for various heat and object sensing applications.
One widely used intrinsic infrared sensitive photodetector material is mercury cadmium telluride, which consists of a mixture of cadmium telluride and mercury telluride. Cadmium telluride is a wide gap semiconductor (Eg = 1.6 eV), and mercury telluride is a semi-metal having a "negative energy gap" of about -0.3 eV. The energy gap of the alloy varies linearly with x, the mole fraction of cadmium telluride in the alloy, Hg.sub.1.sub.-x Cd.sub.x Te. By properly selecting x, it is possible to obtain mercury cadmium telluride detector material having a peak response at any of a wide range of infrared wavelengths.
Mercury cadmium telluride photoconductive detector arrays are presently fabricated by mounting a mercury cadmium telluride crystal on a substrate (e.g. Ge) with an epoxy. The thickness of the mercury cadmium telluride is then reduced to about 10 microns by lapping and etching. The detectors are then delineated and electrically isolated from one another by masking, cutting or etching. Electrical leads are then attached to opposite ends of each of the individual detector elements or to one end of each and a common.
This prior art technique for forming mercury cadmium telluride photoconductive detector arrays has several disadvantages. First, the entire process is quite complex and requires several steps. Second, the epoxy layer, in addition to complicating detector and detector array fabrication, results in a thermal barrier between the mercury cadmium telluride and the substrate. This thermal barrier can adversely affect performance in many device applications. Third, the reduction of thickness of the mercury cadmium telluride to about 10 microns requires considerable precision and is time consuming. Fourth, the isolation by cutting or etching is a time consuming step. Fifth, the electrical leads must be bonded directly to the mercury cadmium telluride detector material. The heat required for bonding can in some cases adversely affect the mercury cadmium telluride detector.